Machinery that is used for processing a moving web, such as of paper or plastic film, must be speed-matched to the entering web. The usual method of doing this is by the use of a dancer, that is a device in which some of the length of the web is stored in a festoon. Conventionally the entering material passes over three or more stationary rollers, with the web festooning between adjacent rollers which rotate about fixed axes (so-called "stationary" rollers) and, at the bottom of the loop, passing around rollers which rotate about axes which can move in a vertical plane (so-called "movable" rollers) and are carried by a pivot arm, which moves up and down as the length of the downwardly descending loops varies in accordance with the material supply requirements of the machine. If the machine speed is too slow, the amount of stored web increases and causes the dancer to move downwardly and, when the machine speed is too fast, the amount of stored web decreases and causes the dancer to move upwardly. It is the usual practice for this dancer to be connected to a potentiometer which controls the machine speed so that an increase in the amount of stored web results in an increase in the machine speed and vice versa. Some machines are intermittent in their transport of the web, stopping the web once each cycle to perform an operation such as punching or sealing and then moving the web faster than the incoming speed to regain the lost travel. In such a machine, the dancer moves back and forth cyclically. If the potentiometer were connected directly to such a dancer, the speed would be adjusted faster and slower each cycle, resulting in excessive wear to the potentiometer and to the drive components. The usual solution to this problem is to use a dead band in connection with the potentiometer, which allows the dancer to travel within a certain limited oscillating range without movement of the potentiometer. If the dancer moves progressively in one direction more than the other, then the dead band is exceeded and a speed correction will be made.
It is common in the art to build a special potentiometer for each different machine in each different dancer. This is expensive and time consuming.